Right now we'll take a look at a startling look at the way ISIS is trying to recruit young Americans. In the wake of the recent terror attack in New York City it is a warning for parents about what most think would never happen in their own homes. Diane sawyer, this is part of your special tonight, "ISIS in America." This really is so shocking. It is shocking and I think people are going to be really surprised by this psychological power of their recruiting techniques and that they're reaching into every American household. We're talking about all religions and all you need is a kid with a computer. A desperate father calls 911 about his son. I need somebody as quick as possible. We have units en route to you. Stay on the line with me, okay. I don't know if it's ISIS or what. This is your son. Yes. You can hear that son in the background. You're going to throw me in jail my whole life. Why are you trying to say I'm a terrorist. Reporter: Justin Sullivan grew up in a catholic home with trophies, friend, a bedroom filled with childhood toys. His dad a retired marine captain rich Sullivan. I imagine him joining the military. And tfollowing my footsteps. Reporter: His high school reported to the parents that Justin had violent fantasies in his writing and suspended him. They said he needed serious help. The parents had no idea that their 18-year-old son alone with his computer had reached out to ISIS recruiters. He sent a message, in less than 24 hours a top captain of ISIS responded. 6,184 miles away. The recruiters seem to know what to do and say to weaponize an isolated kid like Justin. First affirm them like techs from other ISIS case, welcome, brother. Blessings upon you. Justin is lonely. ISIS can fix that. And then another key step. Tap into the fantasies. We know Justin loves the video game call of duty. ISIS has an idea call of jihad. If Justin takes violent action, they will make him a hero, a celebrity. In December 2014 Justin Sullivan puts on a ski mask and murders a neighbor with his father's rifle which he hides. Police find the neighbor's body but have no suspect and then in a few months he tells his ISIS recruiter he's ready to go big. And ISIS says make a videotape so they can be sure he's famous. He's going to get an assault rifle and slaughter people at a concert in North Carolina or at a club. Hundreds will die. He buys a ticket to an upcoming gun show days away from attack. As the clock is ticking down his parents discover a package containing a gun silencer and for the second time the father makes a desperate call for help. I called the police department. I said I need to talk to somebody in homeland security. Reporter: Tonight a father from North Carolina has a warning for parents. He's a marine who fought the enemy abroad. Here I am defending our country against domestic enemies, what so happened to be my son. I didn't think it would be this close to home. By the way you're going to hear from Justin Sullivan who calls us in from prison tonight but I want to remind everybody there are thousands of investigations under way right now in every single state in this country and we've spent a year looking into who's vulnerable and why. This shows just how hard it is to police something like this. A kid alone in his basement. His dad turned him in. A lot of counterterrorism experts are saying this is the new war. ISIS may be losing on the ground, losing territory but this is the new war and we have to pay attention to it. You've been working on this for over a year. A lot of people know what happened here in New York with the truck attack and they thought it was reaction to this but this is something you've been working on for quite some time. This has been a question, a burning question we have had for a long time. The father being aware enough to see this happening and call the authorities on the child. Made all the difference. You all can see the special "20/20," Diane sawyer investigates ISIS in America at 10:00 eastern on ABC.

This transcript has been automatically generated and may not be 100% accurate.

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